The present invention relates generally to imaging systems. More particularly, the present invention relates to an x-ray tube anode with enhanced thermal performance.
Electron beam generating devices, such as x-ray tubes and electron beam welders, operate in a high temperature environment. In an x-ray tube, for example, the primary electron beam generated by the cathode deposits a very large heat load in the anode target to the extent that the target glows red-hot in operation. Typically, less than 1% of the primary electron beam energy is converted into x-rays, while the balance is converted to thermal energy. This thermal energy from the hot target is radiated to other components within the vacuum vessel of the x-ray tube, and is removed from the vacuum vessel by a cooling fluid circulating over the exterior surface of the vacuum vessel. Additionally, some of the electrons back scatter from the target and impinge on other components within the vacuum vessel, causing additional heating of the x-ray tube. As a result of the high temperatures caused by this thermal energy, the x-ray tube components are subject to high thermal stresses which are problematic in the operation and reliability of the x-ray tube.
Typically, an x-ray beam generating device, referred to as an x-ray tube, comprises opposed electrodes enclosed within a cylindrical vacuum vessel. The vacuum vessel is typically fabricated from glass or metal, such as stainless steel, copper or a copper alloy. As mentioned above, the electrodes comprise the cathode assembly that is positioned at some distance from the target track of the rotating, disc-shaped anode assembly. Alternatively, such as in industrial applications, the anode may be stationary.
The target track, or impact zone, of the anode is generally fabricated from a refractory metal with a high atomic number, such as tungsten or tungsten alloy. A typical voltage difference of 60 kV to 140 kV is maintained between the cathode and anode assemblies to accelerate the electrons. The hot cathode filament emits thermal electrons that are accelerated by the potential difference, impacting the target zone of the anode at high velocity. A small fraction of the kinetic energy of the electrons is converted to high energy electromagnetic radiation, or x-rays, while the balance is contained in back scattered electrons or converted to heat. The x-rays are emitted in all directions, emanating from the focal spot, and may be directed out of the vacuum vessel.
In an x-ray tube having a metal vacuum vessel, for example, an x-ray transmissive window is fabricated into the metal vacuum vessel to allow the x-ray beam to exit at a desired location. After exiting the vacuum vessel, the x-rays are directed to penetrate an object, such as human anatomical parts for medical examination and diagnostic procedures. The x-rays transmitted through the object are intercepted by a detector and an image is formed of the internal anatomy. Further, industrial x-ray tubes may be used, for example, to inspect metal parts for cracks or to inspect the contents of luggage at airports.
Since the production of x-rays in an x-ray tube is by its nature a very inefficient process, the components in x-ray generating devices operate at elevated temperatures. For example, the temperature of the anode focal spot can run as high as about 2500.degree. C., while the temperature in the other parts of the anode may range up to about 1800.degree. C. Additionally, the components of the x-ray tube insert must be able to withstand the high temperature exhaust processing of the x-ray tube, at temperatures that may approach approximately 450.degree. C. for a relatively long duration.
To cool the x-ray tube insert, the thermal energy generated during tube operation must be transferred from the anode through the vacuum vessel and be removed by a cooling fluid. The vacuum vessel is typically enclosed in a casing filled with circulating, cooling fluid, such as dielectric oil. The casing supports and protects the x-ray tube and provides for attachment to a computed tomography (CT) system gantry or other structure. Also, the casing is lined with lead to provide stray radiation shielding.
In conventional systems, extra performance from increased heat dissipation is achieved by increasing the diameter and mass of the target to increase the heat storage and radiating surface area of the target. Nevertheless, increasing the diameter and mass of the target is not easily done for the following reasons: (1) Increasing the diameter of the target is limited due to space constraints on the scanning system. Space constraints are particularly applicable to x-ray systems due to the desire to have good angulation capability. (2) Faster scanning on the CT gantry increases the mechanical loads on the entire x-ray tube. Hence, faster scanning tends to drive the mass of the target downward, which conflicts with the thermal performance of the x-ray tube. (3) Thickening the target will provide little benefit for high power scans since there is a finite amount of time required for the heat to conduct from the track of the target (i.e., the region where the electron beam hits the target) to other regions of the target. As such, the heat energy may not even reach the back of the target until the scan has ended. Therefore, adding extra mass to the back of the target will give little to no extra benefit with respect to thermal performance.
Thus, there is a need for an improved method of dissipating heat from the anode of the x-ray tube. Further, there is a need for an x-ray tube which provides greatly enhanced heat dissipation at the track and for the entire target, resulting in the capability to do longer and more powerful x-ray scans. Such an x-ray tube would beneficially operate with lower track temperature. Even further, there is a need for an x-ray tube which provides lower mass and smaller targets for a given power rating, enabling higher gantry speeds on CT systems or better angulation on x-ray systems.